Sins of the Fathers Chapter 1: Doctor Strange - S3-E1
Revealing mistake: When Mary Jane and the other servants of Baron Mordo are flying through the city, they literally disappear into thin air. Probably a problem with rendering the 2D animation within the 3D animation. (00:01:00)
Behold, a Distant Star - S2-E10
Revealing mistake: When the heroes are in the Fantasticar looking for the Skrulls, they fly by the same group of three buildings seven times, and from two opposing camera angles. (00:15:02)
The Defection of Hawkeye - S1-E8
Revealing mistake: When Spider-Woman is blasting at Whirlwind and Blizzard, one shot pans to the left from Whirlwind to Blizzard, and you can see the edge of an animation cel. (00:05:03)